Fire Resistant Steel Structure

ABSTRACT

A fire resistant steel structure ( 10 ) comprises a steel beam ( 14 ) for receiving a vertical load and a support structure for supporting the horizontal beam ( 14 ) at two horizontally spaced locations. At least one fire-resistant tension member ( 30 ) has its ends ( 32, 32 ′) anchored outside the steel beam ( 14 ) in the support structure. It is arranged relative to the steel beam ( 14 ) in such a way that when the steel beam ( 14 ) is overheated and yields under the vertical load in case of severe fire conditions, it rests on the at least one fire-resistant tension member ( 30 ) and is vertically supported by the latter.

TECHNICAL FIELD

The present invention generally relates to a fire resistant steelstructure.

BACKGROUND ART

Unprotected structural steel members like columns, girders, beams etc.lose most of their load bearing capacity when they are exposed totemperatures above 400 C. For warranting the required fire resistancerating in multistorey steel structures, it is well known in the art touse a fireproof heat insulation slowing down temperature rise in loadbearing structural steel components. Known heat insulation measurescomprise e.g.: fireproof encasements with slab-type materials made e.g.from calcium silicate or gypsum; mineral fiber insulations; sprayapplied fireproofing materials and intumescent paints or coatings. Thesefireproof insulations must generally be applied in-situ to all loadbearing structural steel components, which is a costly andtime-consuming operation.

It is also known to use composite profiles, i.e. steel profiles with apartial or full concrete encasement or, alternatively, concrete filledsteel tubes. Such composite profiles have a substantially highermechanical resistance in case of fire than bare steel profiles, i.e.they maintain their load bearing function much longer. However, they arealso much heavier than bare steel profiles, which is a substantialdisadvantage, in particular for horizontal load bearing structural steelmembers, as e.g. beams and girders. (In the following, the term “beam”will be used for designating beams as well as girders.)

A composite profile is e.g. described in EP 1 405 961, which is used tosupport pre-fabricated floor elements. The composite profile comprises aclosed trapezoidal steel section, the inner volume of which is filledwith concrete. The inner volume further comprises a couple of tendonsanchored within the bar and arranged to provide pre-tensioning in such away as to cause a bending moment opposed to that caused by the externalload.

FR 1 544 207 relates to a pre-stressed composite metallic beam. Itcomprises a steel beam having a vertical web extending between twohorizontal flanges. A pair of tie members from a steel with high limitof elasticity are anchored, at both ends of the beam, on the beam web,on both sides thereof, and in the vicinity of the neutral line. The tiemembers are bonded to the web by elements imposing them a curve, thelower point of which is located in the vicinity of the bottom flange, sothat the efforts exerted on these elements and resulting from thetension of the tie members produce deformation stresses in the beam inthe direction opposed to that of the deformation due to the load.

Document US 2007/0028551 describes a beam attachment system comprisingtwo posts and a beam horizontally supported by the two posts. A beam tieis provided to compensate for the stress exerted by the beam on theposts. Accordingly, the beam tie is supported at the head of the postsand engaged in a passage inside the beam to support it. This system isdesigned so that the beam tie compensates, at least partially, for themoment exerted by the beam on the posts and hence retain the stabilityof the system.

TECHNICAL PROBLEM

It is a first object of the present invention to provide a fireresistant steel structure in which a load bearing beam maintains itsload bearing function during the required time of fire exposure withoutnecessarily necessitating costly and time consuming insulation measuresor a heavy concrete encasement or filling.

This object is achieved by a fire resistant steel structure as claimedin claim 1.

It is further object of the present invention to provide a fireresistant steel-concrete floor structure having a good fire resistanceeven without expensive and time consuming insulation measures on theload bearing beams.

This object is achieved by a fire resistant steel-concrete floorstructure as claimed in claim 19.

GENERAL DESCRIPTION OF THE INVENTION

A fire resistant steel structure in accordance with the presentinvention comprises a steel beam for receiving a vertical load and asupport structure for supporting the steel beam at two horizontallyspaced locations (generally but not necessarily at both ends of thesteel beam). At least one fire-resistant tension member, which has itsends anchored outside the steel beam in the support structure, isarranged in relation to the steel beam in such a way that when the steelbeam is overheated and yields under its vertical load in case of severefire conditions, the overheated beam rests on the at least onefire-resistant tension member and is vertically supported by the latter.

It will be appreciated that such an emergency backup support systemwill—by providing an external, collapse retarding catenary supportmechanism for the overheated steel beam—substantially increase the timeduring which a bare steel beam maintains its load bearing function whenit is overheated in case of a fire. It follows that a costly and timeconsuming application of a fireproof insulation onto the steel beam isnot necessary, and that a bare steel beam (i.e. a steel beam withoutfireproof insulation or concrete encasement) may maintain its loadbearing function in case of a fire at least as long as a heavy compositesteel beam (i.e. a steel beam with a partial or full concreteencasement).

For this purpose, the fire-resistant tension member is advantageouslydesigned in such a way as to be able to take essentially all of the loadof the beam that yields during the fire. In other words, thefire-resistant tension member(s) is/are designed to be able to take,under the severe fire conditions, essentially all of the load thatshould be taken by the steel beam (i.e. the load taken by the beamwithout fire—as in the cold state).

Preferably, the tension member shall be able to take during the fire atleast 70%, more preferably at least 80% of the load taken by the beam inthe cold state.

It is to be noted that, as will be explained in more detail below, thefire-resistant tension member may be a tension member having appropriatemechanical performance (in particular an appropriate tensile strength)that is protected against the fire, thus forming a fire-protectedtension member. Alternatively the fire-resistant tension member may be atension member having appropriate mechanical performance (an appropriatetensile strength) and having an inherent good fire resistance, i.e. itkeeps a good tensile strength even at high temperatures (of majorinterest is the range above 600° C., more specifically 600 to 1100° C.).

It will further be appreciated that efficiently using an inherently fireresistant tension member or protecting a slender tension member with afireproof heat insulation is by far easier, less costly and lesstime-consuming than applying such a fireproof heat insulation to thesteel beam itself. Furthermore, such fire-resistant tension membersresult in a smaller surcharge of the support structure than a partiallyencased composite steel beam (with reinforced concrete between theflanges).

In a preferred embodiment, the at least one fire-resistant tensionmember extends along the steel beam, e.g. parallel to a beam web. Inthis embodiment, at least one intermediate support member isadvantageously arranged on the steel beam, in such a way that when theyielding overheated steel beam rests via the at least one intermediatesupport member on the at least one fire-resistant tension member and isvertically supported by the latter. However, the overheated steel beammay also rest directly with a lower flange (or any other beam element)directly on the at least one fire-resistant tension member when ityields under its vertical load.

The at least one intermediate support member arranged on the steel beamis advantageously integrated in a transversal web-stiffener, which ise.g. equipped with a through hole or a cut-out for the at least onefire-resistant tension member. Alternative embodiments of intermediatesupport members comprise e.g. studs or hooks fixed to the beam orcut-outs or holes in an element of the beam itself (as e.g. a flange orweb).

In an optimized embodiment for force transmission between the steel beamand its emergency backup support system, a series of intermediatesupport members are arranged on the steel beam so that the at least onefire-resistant tension member has a polygonal shape approximating aparabola. The more intermediate support members are foreseen, the betterthe fire-resistant tension member approximates the optimal parabolashape and the better force transmission between the steel beam and itsemergency backup support system is. However, for reasons of economy, thesteel beam will most often comprise not more than three intermediatesupport members, which are generally sufficient to warrant the requiredISO fire resistance for the steel beam.

In an alternative embodiment, the emergency backup support system forthe steel beam includes at least one fire-resistant tension memberarranged transversally to the steel beam. When the overheated steel beamyields in this embodiment, it rests on the at least one transversefire-resistant tension member, e.g. directly with its lower flange or bymeans of an intermediate support member. Such a solution with at leastone fire-resistant tension member arranged transversally to the steelbeam may be of particular advantage in combination with a cellular steelbeam having apertures in its web. This is because a transversefire-resistant tension member does not impede the passage of conduitsthrough the apertures in its web of the cellular steel beam.

Preferably, the at least one fire-resistant tension member is onlyslightly pre-stressed when the steel beam is cold, so as to have asufficient reserve for supporting the overheated steel beam. Under themaximum load of the cold beam, the prestress tension in the at least onefire-resistant tension member should preferably not exceed 25%, morepreferably not more than 15% of the tensile strength of the tensionmember. The slight prestress tension shall e.g. warrant that there is nosubstantial play in the anchoring of the ends of the fire-resistanttension member and that the tension member is already in close contactwith beam when a fire breaks out, i.e. that the at least onefire-resistant tension member is capable of developing a catenarysupport mechanism for the overheated beam as soon as the latter beginsto yield in case of a fire. It is however to be noted that thefire-resistant tension member does not need to play a structural role inthe cold state so that it does not need to be pre-stressed. This greatlyfacilitates the installation of such tension members.

In order to increase the fire resistance of existing steel structures,this system is very suitable to be applied for two reasons. First, theinstallation is easy since it requires neither complicate erectionphases nor pre-stressing technology. Second, since it is active only infire condition, it does not require changing the statical functionalityof the structure in cold condition.

In a preferred embodiment, a double-T shaped steel beam with an upperflange, a lower flange and a web connecting the upper flange to thelower flange, comprises on each side of the web, at least onefire-resistant tension member that is anchored in the support structureand extends along the web between the upper flange and the lower flange.Intermediate support members are arranged on both sides of the web,symmetrically in relation to the latter. It follows that when theoverheated steel beam yields under the vertical load, it rests via theintermediate support members on the fire-resistant tension members andis vertically supported by the latter symmetrically in relation to theweb.

The steel beam is preferably supported by the support structure in sucha way that it may axially expand when heating up under severe fireconditions, whereby excessive compressive axial forces in the beam,which may cause a buckling of the latter, are avoided.

The support structure may comprise a H-shaped steel column with twoflanges and a concrete filling between the flanges, wherein a first endof the steel beam is fixed to a first of the two column flanges, and oneend of the fire-resistant tension member passes through a through-holein this first column flange and is provided with an anchoring elementthat is embedded in the concrete filling between the column flanges.

A first embodiment of the at least one fire-resistant tension member, itmay advantageously comprise a high strength steel strand (or anyequivalent tension member) provided with an envelope filled with afireproof mortar or grout.

The at least one fire-resistant tension member may alternativelycomprise a high strength steel strand (or any equivalent tension member)provided with an intumescent coating, an intumescent paint, anintumescent sleeve, a sprayed fire insulating material or a fireproofinsulation sleeve.

Besides, as previously mentioned, the fire-resistant tension member maybe a tension member having appropriate tensile strength and having aninherent good fire resistance. In this case, one may use any appropriatematerial, presently existing or to be developed, having a tensilestrength which does not severely drop even at high temperatures, namelyabove 600° C.; the material may be in the form of tendons, wires, strandor fibers that may be assembled to form a tension member of largersection.

Those skilled in the art may identify suitable non-metallic materials,namely synthetic materials having a high elastic limit and showing goodfire resistance, and typically materials allowing the manufacture of atension member with a tensile strength of at least 500 MPa at hightemperatures (above 600° C.).

The present invention also provides a fire resistant steel-concretefloor structure comprising a concrete slab and a support structure forthe concrete slab including at least one steel beam. At least onefire-resistant tension member having its ends anchored outside the steelbeam in the slab is arranged relative to the steel beam in such a waythat when said steel beam is overheated and yields under its load incase of severe fire conditions, the overheated beam rests on the atleast one fire-resistant tension member and is vertically supported bythe latter, the fire-resistant tension member being designed to be ableto take essentially all of its load.

The at least one fire-resistant tension member has its ends anchored inthe concrete slab advantageously in direct vicinity of a support columnor an other vertical support member. This warrants that the tensileforce in the tension member exerts no significant bending moment ontothe steel beam or the slab.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way ofexample, with reference to the accompanying drawings in which:

FIG. 1 is a cross-sectional view of a fire resistant steel structurewith a steel beam equipped with an emergency support system comprising afire-resistant tension member;

FIG. 2 is diagrammatic view of a fire resistant steel structure as shownin FIG. 1, showing—in the absence of fire—the bending moment in thesteel beam and the axial force in the fire-resistant tension member;

FIG. 3 is diagrammatic view as in FIG. 2, showing—under fireconditions—the bending moment in the overheated steel beam and the axialforce in the fire-resistant tension member;

FIG. 4 is a diagram illustrating, during an ISO fire exposure, the loadbearing mechanism of a steel beam equipped with an emergency support;the numbers on the x-coordinate represent the time of ISO fire exposurein seconds (s) and the numbers on the y-coordinate represent the portionof the load taken by the steel beam and the tension member in percent(%);

FIG. 5 is a diagram comparing the deflection of an of an unprotectedsteel beam without fire-resistant tension members, an unprotected steelbeam with fire-resistant tension members interacting with twointermediate support members and an unprotected steel beam withfire-resistant tension members interacting with three intermediatesupport members; the numbers on the x-coordinate represent the time ofISO fire exposure in seconds (s), and the numbers on the y-coordinaterepresent the deflection of the steel beam in meters (m);

FIG. 6 is a cross-sectional view illustrating first anchoring of afire-resistant tension member on a column;

FIG. 7 is a cross-sectional view illustrating an anchoring of afire-resistant tension member in a slab;

FIG. 8 is a cross-sectional view illustrating an alternative anchoringof a fire-resistant tension member on a column; and

FIG. 9 is a cross-sectional view of an embodiment of a fire-protectedtension member.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a fire resistant steel structure 10 in accordance with theinvention. This steel structure 10 comprises two columns 12, 12′ (i.e.vertical structural members) forming a support for a steel beam 14 (i.e.a horizontal structural member) at two horizontally spaced locations.The steel beam 14 serves as support element for a slab 16, in this casee.g. a concrete slab with profiled steel sheets.

The columns 12, 12′ shown in FIG. 1 are H-shaped steel beams with areinforced concrete filling 18 between the flanges (such columns aregenerally called composite columns). The reinforced concrete filling 18warrants that the columns 12, 12′ maintain their load bearing functionduring the required time of fire exposure. Alternatively, the columns12, 12′ might also be steel profiles protected with a fireproof casingmade e.g. of silicate or gypsum plates with or without mineral fiberinsulations, with spray applied fireproofing materials, intumescentpaints or coatings, respectively steel profiles completely encased inconcrete or closed steel profiles completely filled with concrete. Thecolumns 12, 12′ may also be steel reinforced concrete columns or woodencolumns or they may be replaced by other suitable support elements forthe steel beam, as e.g. a concrete wall or a brick wall.

The steel beam 14 shown in FIG. 1 is a double-T-shaped steel beam (or anI-beam), i.e. a steel beam having an upper flange 20, a lower flange 22and a vertical web 24. It will be noted that the steel beam 14 as suchis not provided with a passive fire protection, at least not with apassive fire protection substantially increasing its load bearingcapacity under fire exposure. Consequently, when the steel beam 14 assuch is exposed to a severe fire (i.e. a fire resulting in temperaturesof the steel beam above 400° C.), it will generally not resist more thanhalf an hour before collapsing under its load.

It will be noted that the steel beam 14 is supported between the columns12, 12′ in such a way that it may axially expand when heating up undersevere fire conditions, thereby avoiding excessive compressive axialforces in the steel beam 14. Such a free expansion can easily beimplemented by providing e.g. a double web cleated connection (asidentified e.g. with reference number 26 in FIG. 1) or a fin plateconnection (not shown) between the web 24 of the steel beam 14 and aflange 28 of the column 12, wherein fixing bolts on the side of the web24 may horizontally slide within oblong bolt holes when the steel beam14 expands.

Reference number 30 identifies a fire-resistant tension member, whichforms an emergency backup support system for the steel beam 14, when thelatter yields under severe fire conditions. This fire-resistant tensionmember 30 has its ends anchored outside said steel beam 14. The firstend 32 of the fire-protected tension member 30 is e.g. equipped with ananchor 34 cooperating with the flange 28 of the column 12 for anchoringit on the column 12, and the second end 32′ is e.g. equipped with ananchor 34′ cooperating with the flange 28′ of the column 12′ foranchoring it on the column 12′. In accordance with a general principleunderlying the present invention, the fire-resistant tension member 30is arranged in such a way that when the overheated steel beam 14 yieldsunder its vertical load in case of severe fire conditions, it rests onthe fire-resistant tension member and is vertically supported by thelatter. Due to its fire resistance, the tension member 30 keeps its loadbearing capacity longer than the unprotect beam 14. The tension member30 is thus advantageously designed to be able to take essentially all ofthe load of the beam under the severe fire conditions.

It will be appreciated that using a fire-resistant tension member is byfar less costly and time-consuming than providing a passive fireprotection to the steel beam 14 itself.

Hence, under fire conditions, the tension member or members willprogressively take up the load that is no longer taken by the yieldingbeam, and the design of the tension member is made so as to be able tosupport essentially all of the weight of the beam together with thevertical load received by the beam, and this during the fire. This ispossible, firstly, since the tension member 30, respectively the groupof tension members, is/are: (a) dimensioned to be able to takeessentially all of the load taken by the beam (preferably at least 70%,more preferably at least 80%, or possibly up to nearly 100%). Andsecondly because the tension member(s) is/are fire resistant, either bythe help of a protective coating or due to inherent fire resistance ofthe material from which the tension member is made. As it is known, thetensile strength (rupture point) of a metallic material is temperaturedependent. What matters here is that the fire-resistant tension membersbe able to withstand the load supported by the beam (and of the beamitself) during a certain time of fire exposure. The materials for thetension members, and the possible amount of fire protection, is thus tobe selected keeping this aspect in mind. It is however clear that thewhen exposed to fire, the tensile strength of the tension members mayhowever decrease, but still remain at a level sufficient to bear theload of the beam.

In summary, a tension member, whether protected or inherently fireresistant, shall advantageously be designed so as to be able to keep abearing capacity at room temperatures above 600° C., more preferably inthe range of 600° C. to 1100° C., sufficient to take essentially all ofthe load of the yielding beam. Room temperatures between 600° C. and1100° C. are in Civil Engineering typically the consequence of a severefire.

-   -   At the design stage of the tension member, as first simplified        approach, one may determine the section of the tension member        based on the load borne by the beam, and therefore use the        following, well known formulae:

$S = {{\frac{F}{T_{S}}\mspace{14mu} {and}\mspace{14mu} F} = \frac{Q \cdot l^{2}}{d \cdot 8}}$

where F is the force in the tension member; Q is the load on the beam(the load of the beam itself is actually negligible but can be takeninto account in Q); l is the span of the beam; d is the distance betweenthe top and low points of the tension member along the beam (function ofthe initial given shape and of the beam deflection); S is the section ofthe tension member and Ts is the tensile strength (rupture point). Ofcourse, one shall use a coefficient of security in these calculations.

In the preferred embodiment shown in FIG. 1, the fire-resistant tensionmember 30 extends parallel to the steel beam 14 between the upper flange20 and the lower flange 22. When the overheated steel beam 14 yieldsunder its vertical load in case of overheating in a fire, it rests bymeans of intermediate supports 38 ₁ and 38 ₂ on the fire-resistanttension member 30. Such intermediate supports 38 ₁ and 38 ₂ areadvantageously transversal web stiffeners as shown in FIG. 1, each ofthem having a through hole or cut-out therein through which thefire-resistant tension member 30 passes. Other possible embodiments ofsuch intermediate supports 38 ₁, 38 ₂ are e.g. studs or hooks (welded orbolted directly to the web 24 or to the lower or upper flange 22) or areformed by cut-outs or holes in the lower flange 22 of the steel beam 14.It will be appreciated that intermediate supports 38 ₁, 38 ₂incorporated in robust transversal web stiffeners (as shown in FIG. 1)have the advantage of being less exposed to the risk of becomingprematurely ineffective due to a local buckling of the overheated steelbeam 14. The intermediate supports shall preferably be fire-resistant aswell and may hence advantageously be provided with a fireproof heatinsulation as e.g. a fireproof casing, a spray applied fireproofingmaterial or an intumescent paint. In order to even further increase theefficiency of the emergency backup support system for the steel beam 14with relatively low additional costs, the transversal web stiffeners maybe provided with a fireproof heat insulation too.

It will be appreciated that force transmission in the emergency backupsupport system for the overheated steel beam 14 may be optimized byproviding a series of such intermediate supports on the steel beam 14,wherein these intermediate supports are advantageously arranged so thatthe fire-resistant tension member 30 has a polygonal shape approximatingmore or less a flat parabola, with its minimum near the lower flange inthe middle of the steel beam 14. Furthermore, if the steel beam 14 has aweb 24 and a vertical plane of symmetry (such as e.g. a double-T beam asshown in FIG. 1), the emergency backup support system for the steel beam14 preferably comprises at least one fire-resistant tension member 30and at least one intermediate support arranged on each side of the web24, so as to be symmetric in relation to the latter. When the overheatedsteel beam 14 yields under its vertical load, it is vertically supportedby the fire-protected tension members on both side of the web 24 andthis symmetrically in relation to the latter, thereby reducing the riskof an asymmetrical deformation of the overheated steel beam, which couldresult in a premature collapse.

Instead of having fire-resistant tension members 30 extending betweenthe upper flange 20 and the lower flange 22 parallel to the web of thesteel beam 14, the emergency backup support system for the overheatedsteel beam 14 could also include one or more fire-resistant tensionmembers (not shown) arranged transversally to the steel beam 14, whereinthe overheated steel beam 14 could e.g. rest directly with its lowerflange on the fire-resistant tension member. Such an arrangement oftransverse fire-resistant tension members could support more than onesteel beam. It may be of particular advantage when used in combinationwith cellular steel beams.

For computing the diagrams of FIGS. 2, 3, 4 and 5, finite elementcalculations have been performed with the software SAFIR developed bythe University of Liege (Belgium). The steel beam 14 is an unprotectedIPE 500 beam with a span of 12 m. At each end of the steel beam 14,vertical and lateral displacements are blocked. The right end of thesteel beam 14 is however free to expand horizontally. There are twofire-resistant tension members 30 arranged symmetrically in relation tothe web 24 and actually taking each the form of a fire-protected steelcable. Each of these tension members 30 is fully restrained at both endsin an independent support structure, i.e. the tension in the tensionmember 30 does not affect (i.e. compress) the steel beam 14.Consequently, in contrast to known external unbonded tendons used inconcrete beams or steel/concrete composite beams, the tension members 30do not significantly compress the steel beam 14, neither in the coldstate nor in the hot state. The tension members 30 are considered toremain cold during the whole fire, and the intermediate supports aresupposed to be designed in such a way that they are not affected bypremature local collapse or buckling of the steel beam 14. For thecalculation it has been assumed that the steel beam 14 supports adistributed loading of 30 kN/m, that the yield strength of the steelbeam 14 is 355 MPa, and the tensile strength of each tension member 30is 1860 MPa.

Referring now to FIGS. 2 and 3, the working principle of the steel beam14 with its emergency backup support system will be described. It willfirst be noted that FIG. 2 shows the steel beam 14 under a uniformlydistributed load in the absence of fire (i.e. in a cold state), and FIG.3 shows the same steel beam 14 when it has already substantially yieldedunder its uniformly distributed load due to overheating in case of afire (i.e. when the steel beam 14 has e.g. reached a temperature above600° C.). The two anchoring points of the fire-protected tension member30 are identified with reference numbers 36 and 36′. For each tensionmember 30 there are two intermediate supports 38 ₁ and 38 ₂, the firstone located at 4 m from the first beam support the second one located at4 m from the first beam support. It remains to be noted that the bendingmoment and tensile force diagrams in FIGS. 2 and 3 are drawn in the samescale.

In FIG. 2, reference number 40 identifies the bending moment diagram forthe steel beam 14 in the cold state, i.e. when it still has its wholeload bearing capacity. This diagram 40 is a well-known parabolic bendingmoment diagram for a uniformly charged beam having at one end a pin-typesupport 42 and at the other end a roller-type support 44. Referencenumber 46 identifies the tensile force diagram for the fire-protectedtension member 30. It will be noted that in the cold state of the steelbeam 14, the fire-protected tension member 30 is subjected to a smallprestress, which is sufficient to warrant that there is no substantialplay in the anchoring points 36 and 36′, and that the tension member 30is in close contact with the intermediate supports 38 ₁ and 38 ₂. Thisprestress tension is only a small percentage (i.e. generally less than15%) of the tensile strength of the tension member 30 at cold state.

In FIG. 3, reference number 40′ identifies the bending moment diagramfor the overheated steel beam 14, that is when it has itself only asmall remaining load bearing capacity. The overheated steel beam 14,which has yielded under its load, rests now with its intermediatesupports 38 ₁ and 38 ₂ on the fire-protected tension members 30,which—due to their fire protection—have conserved all their tensileforce and bearing capacity. The bending moment diagram 40′ for the steelbeam 14 is now a typical diagram for a uniformly charged beam having atone end a pin-type support 42 and at the other end a roller-type support44 and, between these two end supports, two intermediate supports 38 ₁and 38 ₂, which rest on the fire-protected tension members 30. Due tothe intermediate supports 38 ₁ and 38 ₂, the maximum moment to which thesteel beam 14 is exposed is considerably reduced (roughly by a factor10), so that the steel beam 14, which is considerably weekend byoverheating, may still support the reduced moment to which it isexposed. The tensile force diagram 42′ for the fire protected tensionmember 30 shows that the tensile force in the tension member 30 hassubstantially increased. This increase causes no problem because thefire-protected tension member 30 is still relatively cold, so that ithas nearly its full tensile force bearing capacity for which it has beendesigned. It will be noted in this context that a high-strength steelstrand, as e.g. a seven wires-strand with an equivalent diameter of 15.7mm, may easily have a rupture limit above 1800 MPa. In conclusion, witha small diameter commercial steel strand equipped with a suitable fireprotection, it will be possible to prevent the unprotected beam 14 froma premature collapse.

FIG. 4 is a diagram illustrating the load bearing mechanisms of a steelbeam equipped with an emergency backup support system during an ISO fireexposure, i.e. using the time/temperature curve ISO 834 for simulatingthe fire. The numbers on the x-coordinate represent the time of ISO fireexposure in seconds (s), and the numbers on the y-coordinate representthe portion of the bearing capacity taken by the steel beam 14 and thetension members 30 in percent (%). The curve 48 shows how the load takenby the steel beam 14 decreases and the curve 50 how the load taken bythe fire-protected tension members 30 increases with fire exposure time.Initially, the steel beam 14 takes nearly 100% of the load. After 15minutes (900 s) of ISO fire exposure, the steel beam 14 takes 70% of theload, and the fire-protected tension member takes the complementary loadno longer taken by the beam (30%). After about 32 minutes (1920 s) ofISO fire exposure, the situation is reversed, i.e. the steel beam 14takes now 30% of the load and the fire-protected tension member 70%.Arrow 52 identifies an initial time sector of about 15 minutes duringwhich the bending resistance of the steel beam 14 prevails. Arrow 53identifies a transient phase during which the bending resistance of thesteel beam 14 becomes less important than the resistance of thefire-protected tension member 30, and arrow 54 a phase during which acatenary load carrying mechanism prevents a collapse of the unprotectedbeam until 60 minutes (3600 s) of ISO fire exposure. It will beappreciated that this catenary load carrying mechanism optimally usesstrength reserves of the steel in the steel beam 14 and in thefire-protected tension members 30.

FIG. 5 is a diagram comparing, for an ISO 834 time/temperature curve,the deflection: (1) of an unprotected steel beam without fire-protectedtension members (see curve 56); (2) of an unprotected steel beam withfire-protected tension members interacting with two intermediate supportmembers (see curve 57); and (3) of an unprotected steel beam withfire-protected tension members interacting with three intermediatesupport members (see curve 58). The numbers on the x-coordinaterepresent the time of ISO fire exposure in seconds (s), and the numberson the y-coordinate represent the deflection of the steel beam in meters(m). The unprotected steel beam without fire-protected tension membersloses its bearing capacity in less than 10 minutes (see curve 56). Theunprotected steel beam with fire-protected tension members interactingwith two intermediate support members maintains its bearing capacity forone hour (see curve 57). The unprotected steel beam with fire-protectedtension members interacting with three intermediate support membersmaintains its bearing capacity even for more than two hours (see curve58).

In summary, it has been seen that the tension member, or group oftension members, are designed and arranged in such a way to be able tosupport the beam and its load under severe fire conditions (typically athigh temperatures above 600° C. and preferably in the range of 600° C.to 1000° C.), for a desired exposure time.

The required load bearing capacity for the tension member(s) isdetermined from the load to be supported by the beam and the beam weightin cold conditions. And the tension member(s) are thus able to withstandthis load under severe fire conditions, due to the fact that they arefire protected or made from a material having inherently good fireresistance. In other words, the tensile strength of the tension members,during fire exposition, is still sufficient to support essentially allof the load, preferably at least 70%, more preferably least 80% of theload constituted by the beam and the load supported by the latterwithout fire.

In addition, the number of intermediate supports has an incidence on theresistance of the structure over time during ISO testing. The number ofintermediate supports is advantageously designed in such a way to reachthe desired fire resistance of the system. As a simplified approach, thebeam 14 with the tension members 30 can be considered as a continuousgirder over x supports, where x−2 is the number of intermediate supportsprovided by the tension member deviation device. In praticalapplications, two or three deviators will be sufficient for most of thecases.

FIGS. 6, 7 and 8 illustrate three embodiments of the anchoring of thefire-protected tension member 30. In the embodiment of FIG. 6, the end32 of the tension member 30 passes through a hole in the flange 28 ofthe column 12 and is secured by means of an anchor 34 on the inside ofthe flange 28. Here, the anchor 34 and the end 32 of the tension member30 are embedded in a concrete filling 60 put in place in-situ in betweentwo transversal stiffener plates 62, 64 of the column 12. This concretefilling 60 slows down heating-up of the anchoring of the fire-protectedtension member 30 in case of a fire. In the embodiment of FIG. 7, theend 32 of the tension member 30 passes through a hole in the upperflange 20 of the steel beam 14 and is anchored within the slab 16,preferably within a slab reinforcement 66. Here, the concrete of theslab 16 slows down heating-up of the anchoring of the fire-protectedtension member 30 in case of a fire. It will be noted that the anchoringin the slab 16 is located close to the supporting column 12, so that—incase of fire—the considerable tensile force in the tension member 30exerts no significant bending moment onto the steel beam 14 or the slab16. In the embodiment of FIG. 8, the end 32 of the tension member 30passes through a hole in flange 28 and a hole in flange 29 of the column12. Here, this end 32 is secured by means of an anchor 34 on the outsideof the flange 29. The anchor 34 and the end 32 of the tension member 30are embedded within the slab 16, which slows down their heating-up incase of fire.

It may be noted that a tension member may be associated with severalaligned beams, in which case it may be supported in the two columnsdirectly neighboring the beam, but the tension member may still beextended and pass through one column to support the next beam and so on.In such case, the tension member may be anchored only in the extremitycolumns.

FIG. 9 is a cross-section of a first embodiment of a fire-protectedtension member, i.e. a tension member provided with a fire protection toensure fire resistance. The tension member 70 itself is advantageously asteel strand, e.g. a seven-wire, uncoated steel strand, such as usede.g. in pre-tensioned and post-tensioned prestressed concreteconstructions. Reference number 72 points to an envelop, made e.g. of afireproof material, delimiting an annular space around the tensionmember 70. This annular space is filled with high-pressure fireproofgrout or mortar 74, which should have good heat insulation qualities inorder to obtain the required ISO fire rating for the fire-protectedtension member without having an isolation that is too big. Analternative embodiment of a fire-protected tension member consists e.g.of a high strength steel strand provided with an intumescent coating orpaint or an intumescent or fire insulating sleeve or a sprayed fireinsulating material. Instead of using a high strength steel strand astension member, one might also use traction cables or slender tractionbars. However, with their high tension strength, commercial steelstrands are probably the most suitable product for the present use.

The skilled person may select for the tension members other materialshaving an appropriate tensile strength to take the load of the beam andhaving a better fire resistance, to be used with or withoutfire-protective coating.

For example, fire resistant steel may be used. Fire-resistant steelshave been widely developed in Japan or in Germany and their specifity isto keep a significant percentage of their tensile strength even at hightemperatures. For example, they can have still 93% of the initialtensile strength until 600° C. For increased safety, a fire-resistivecoating may however still be used. Stainless steel may e.g also be used,preferably with a fire-resistive coating.

Those skilled in the art may alternatively identify suitablenon-metallic materials, namely synthetic materials having a high tensilestrength and showing an intrinsic good fire resistance, capable withoutfire protection of taking the load under severe fire conditions.

Legend: 10 steel structure 12, columns 12′ 14 steel beam 16 slab 18concrete filling of 12, 12′ 20 upper flange of 14 22 lower flange of 1424 vertical web of 14 26 double web cleated connection 28 flange of 1229 flange of 12 30 fire-protected tension member 32 first end of 30 32′second end of 30 34 anchor on 32 34′ anchor on 32′ 36, anchoring pointsof 30 36′ 38₁, intermediate supports 38₂ 40 bending moment diagram for14 (cold state) 40′ bending moment diagram for 14 (hot state) 42pin-type support 44 roller-type support 46 tensile force diagram for 30(cold state) 46′ tensile force diagram for 30 (hot state) 48 curve inFIG. 4 50 curve in FIG. 4 52 arrow in FIG. 4 53 arrow in FIG. 4 54 arrowin FIG. 4 56 curve in FIG. 5 57 curve in FIG. 5 58 curve in FIG. 5 60concrete filling 62, stiffener plates of 12 64 66 reinforcing steels of16 70 high strength steel strand 72 envelop of 70 74 grout or mortar

1-20. (canceled)
 21. A fire resistant steel structure comprising: asteel beam for receiving a vertical load; a support structure forsupporting said horizontal beam at two horizontally spaced locations;and at least one fire-resistant tension member having its ends anchoredoutside said steel beam in said support structure and being arranged inrelation to said steel beam in such a way that when said steel beam isoverheated and yields under said vertical load in case of severe fireconditions, said overheated beam rests on said at least onefire-resistant tension member and is vertically supported by the latter,wherein said at least one fire-resistant tension member is designed insuch a way as to be able to take essentially all of the load of saidsteel beam.
 22. The fire resistant steel structure according to claim21, wherein said at least one fire-resistant tension member isdimensioned in such a way as to be able to take at least 70%, preferablyat least 80% of the load.
 23. The fire resistant steel structureaccording to claim 21, wherein said at least one fire-resistant tensionmember does not significantly compress the steel beam.
 24. The fireresistant steel structure according to claim 21, wherein the pre-tensionin said at least one tension member is not more than 25%, preferably notmore than 15% of its tensile strength.
 25. The fire resistant steelstructure according to claim 21, wherein said steel beam has a lowerflange with which said overheated beam rests on said at least onefire-resistant tension member and is vertically supported by the latter.26. The fire resistant steel structure according to claim 21, whereinsaid steel beam is a cellular beam with a web having apertures therein;and said at least one fire-resistant tension member is arrangedtransversally to said steel beam.
 27. The fire resistant steel structureaccording to claim 21, wherein said steel beam is supported by saidsupport structure in such a way that it may axially expand when heatingup under severe fire conditions, thereby avoiding excessive compressiveaxial forces therein.
 28. The fire resistant steel structure accordingto claim 21, wherein said at least one fire-resistant tension member isa fire-protected tension member.
 29. The fire resistant steel structureaccording to claim 28, wherein said at least one fire-resistant tensionmember comprises a high strength steel strand provided with an envelopefilled with a fire insulating mortar or grout.
 30. The fire resistantsteel structure according to claim 28, wherein said at least onefire-resistant tension member comprises a high strength steel strandprovided with an intumescent coating or paint or an intumescent sleeveor a sprayed fire insulating material.
 31. The fire resistant steelstructure according to claim 21, wherein said at least onefire-resistant tension member comprises one or more strands of materialhaving an appropriate tensile strength under severe fire conditions,without fire-protective coating.
 32. A fire resistant steel structurecomprising: a steel beam for receiving a vertical load; a supportstructure for supporting said horizontal beam at two horizontally spacedlocations; at least one fire-resistant tension member having its endsanchored outside said steel beam in said support structure and extendingalong said steel beam; and at least one intermediate support memberarranged on said steel beam in such a way that said overheated steelbeam rests via said at least one intermediate support member on said atleast one fire-resistant tension member and is vertically supported bythe latter, when said steel beam is overheated and yields under saidvertical load in case of severe fire conditions.
 33. The fire resistantsteel structure according to claim 32, wherein said at least oneintermediate support member arranged on said steel beam is integrated ina transversal web-stiffener.
 34. The fire resistant steel structureaccording to claim 32, wherein said at least one intermediate supportmember arranged on said steel beam is a stud, hook or eye arranged onsaid steel beam.
 35. The fire resistant steel structure according toclaim 32, wherein a series of intermediate support members are arrangedon said steel beam so that said at least one fire-resistant tensionmember has a polygonal shape approximating a parabola.
 36. The fireresistant steel structure according to claim 32, wherein said at leastone fire-resistant tension member is slightly pre-stressed when saidsteel beam is not exposed to a fire.
 37. The fire resistant steelstructure according to claim 32, wherein said at least onefire-resistant tension member is a fire-protected tension member. 38.The fire resistant steel structure according to claim 32, wherein saidat least one fire-resistant tension member comprises one or more strandsof material having an appropriate tensile strength under severe fireconditions, without fire-protective coating.
 39. A fire resistant steelstructure comprising: a double-T shaped steel beam for receiving avertical load, said double-T shaped steel beam having an upper flange, alower flange and a web connecting said upper flange to said lowerflange; a support structure for supporting said beam at two horizontallyspaced locations; on each side of said web, at least one fire-resistanttension member anchored in said support structure and extending alongsaid web between said upper flange and said lower flange; andintermediate support members arranged on said steel beam, symmetricallyin relation to said web, in such a way that said overheated steel beamrests via said intermediate support members on said fire-resistanttension members and is vertically supported by the latter symmetricallyin relation to said web, when said steel beam is overheated and yieldsunder said vertical load in case of severe fire conditions.
 40. The fireresistant steel structure according to claim 39, wherein said steel beamis supported by said support structure in such a way that it may axiallyexpand when heating up under severe fire conditions, thereby avoidingexcessive compressive axial forces therein.
 41. The fire resistant steelstructure according to claim 39, wherein said support structurecomprises a H-shaped steel column with two flanges and a concretefilling between said flanges, wherein a first end of said steel beam isfixed to a first of said two flanges; and one end of said fire-resistanttension member passes through a through-hole in said first flange and isprovided with an anchor that is embedded in the concrete between saidcolumn flanges.
 42. The fire resistant steel structure according toclaim 39, wherein said at least one fire-resistant tension member is afire-protected tension member.
 43. The fire resistant steel structureaccording to claim 39, wherein said at least one fire-resistant tensionmember comprises one or more strands of material having an appropriatetensile strength under severe fire conditions, without fire-protectivecoating.
 44. A fire resistant steel-concrete floor structure comprising:a concrete slab; a support structure for said concrete slab including atleast one steel beam; and at least one fire-resistant tension memberhaving its ends anchored outside said steel beam in said slab and beingarranged relative to said steel beam in such a way that when said steelbeam is overheated and yields under its load in case of severe fireconditions, said overheated beam rests on said at least onefire-resistant tension member and is vertically supported by the latter,said at least one fire-resistant tension member being designed in such away as to be able to take essentially all of the load of said beam. 45.The fire resistant steel-concrete floor structure according to claim 44,wherein said support structure for said concrete slab comprises supportcolumns; and said at least one fire-resistant tension member has itsends anchored in said concrete slab in direct vicinity of said supportcolumns.